Largely on account of their high speed, high current driving capability and low 1/f noise, heterojunction bipolar transistors (HBTs) are being used as integrated switching devices and microwave devices, e.g., in wireless communications, satellite direct broadcast systems, automobile collision avoidance systems, global positioning systems, and other high-frequency applications.
An HBT can be formed, e.g., at a surface of a semi-insulating gallium arsenide substrate, with a heavily silicon-doped (n-type) sub-collector gallium arsenide layer on the substrate, a lightly silicon-doped (n-type) gallium arsenide collector layer on the sub-collector layer, a carbon-doped (p-type) gallium arsenide base layer on the collector layer, a silicon-doped (n-type) aluminum gallium arsenide emitter layer on the base layer, and with metallic contacts to the emitter, base and sub-collector layers. Conventionally, the base contact metal, having an area which is greater than the emitter area, overlaps with the sub-collector layer. This overlap contributes a major share of the extrinsic base-collector capacitance, towards the total base-collector capacitance. Such an HBT structure is disclosed in U.S. Pat. No. 5,286,997, issued Feb. 15, 1994 to D. Hill.
High-frequency performance of HBTs, in microwave applications, requires reduced extrinsic base-collector capacitance. To this end, M. R. Frei et al., "Selective Growth of InGaAs/InP Heterojunction Bipolar Transistors with a Buried Subcollector", Applied Physics Letters 61, 1992, pp. 1193-1195 discloses selective growth of a buried sub-collector having minimized overlap with the base contact layer. However, this technique has been found difficult to implement in practice.
Another known technique for reducing the extrinsic base-collector capacitance makes use of hydrogen ion implantation into the sub-collector layer, as disclosed by M.-C. Ho et al., "High-Performance Low-Base-Collector Capacitance AlGaAs/GaAs Heterojunction Bipolar Transistors Fabricated by Deep Ion Implantation", IEEE Electron Device Letters 16, 1995, pp. 512-514. Since ion implantation is through the base contact region, the base resistance is increased in the process, which tends to degrade the microwave performance.